The present invention is directed to integrated circuits and, more particularly, to a voltage level shifter.
Complex semiconductor integrated circuits (ICs) often have logic circuits and/or signal and data processors formed of transistors, such as metal-oxide field-effect transistors (MOSFETs) that have a fast operating speed and occupy a small area of the semiconductor die. However, such transistors have a limited breakdown voltage, the maximum voltage that the transistors can withstand without breaking down, particularly the voltage difference that can be applied across the gate insulator of a MOSFET and its source (or drain).
It is possible to include in a single IC die both transistors with small feature size and low breakdown voltages in a low voltage domain and transistors with higher breakdown voltages in a high voltage domain, provided that the low breakdown voltage transistors are not exposed to voltages of the high voltage domain. The high voltage domain is often floating, that is to say that the two voltage domains do not have a common power supply voltage. A voltage level shifter typically receives an input signal from a low voltage domain and transfers the signal to a high voltage domain. Applications for a voltage level shifter include a brushless direct current (DC) motor driver, and a DC-DC voltage converter, for example.
A voltage level shifter is exposed to both the low and high voltage domains. Transistors of the level shifter that are exposed to high voltage difference need high breakdown voltage characteristics. However the precautions taken to protect a high voltage transistor from breakdown greatly increase the die area occupied, especially for ultra-high voltage (UHV) MOSFETs capable of withstanding breakdown voltages of 5V or more. Therefore, it is desirable to reduce the number of high breakdown voltage transistors in a voltage level shifter.